Interferometry method for ellipsometry, reflectometry, and scatterometry measurements, including characterization of thin film structures

ABSTRACT

A method including: imaging test light emerging from a test object over a range of angles to interfere with reference light on a detector, wherein the test and reference light are derived from a common source; for each of the angles, simultaneously varying an optical path length difference from the source to the detector between interfering portions of the test and reference light at a rate that depends on the angle at which the test light emerges from the test object; and determining an angle-dependence of an optical property of the test object based on the interference between the test and reference light as the optical path length difference is varied for each of the angles.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of prior U.S. patent application Ser.No. 11/760,163, filed Jun. 8, 2007, now U.S. Pat. No. 7,403,289, whichis a continuation of prior U.S. patent application Ser. No. 11/542,617,filed Oct. 3, 2006, now U.S. Pat. No. 7,315,382, which is a continuationof prior U.S. patent application Ser. No. 10/659,060, filed Sep. 9,2003, now U.S. Pat. No. 7,139,081, which, in turn, claims priority toeach of: U.S. Provisional Patent Application Ser. No. 60/409,147 filedSep. 9, 2002 and entitled “Back-Focal Plane Ellipsometry, Reflectometryand Scatterometry By Fourier Analysis Of Vertically-Scanned InterferenceData;” U.S. Provisional Patent Application Ser. No. 60/452,615 filedMar. 6, 2003 and entitled “Profiling Complex Surface Structures UsingHeight Scanning Interferometry;” and U.S. Provisional Patent ApplicationSer. No. 60/478,300 filed Jun. 13, 2003 and entitled “ScanningInterferometry.” The contents of the prior applications are incorporatedherein by reference in their entirety.

BACKGROUND

The invention relates to surface topography measurements of objectshaving thin films or discrete structures of dissimilar materials. Suchmeasurements are relevant to the characterization of flat panel displaycomponents, semiconductor wafer metrology, and in-situ thin film anddissimilar materials analysis.

Ellipsometry can be used to analyze the optical properties of a complexsurface. Ellipsometry relies on the difference in complex reflectivityof a surface when illuminated at an oblique angle, e.g. 60°, sometimeswith a variable angle or with multiple wavelengths. Many types ofellipsometer are known in the art.

To achieve greater resolution than is readily achievable in aconventional ellipsometer, microellipsometers measure phase and/orintensity distributions in the back focal plane of the objective, alsoknown as the pupil plane, where the various illumination angles aremapped into field positions. Such devices are modernizations oftraditional polarization microscopes or “conoscopes,” linkedhistorically to crystallography and mineralogy, which employs crossedpolarizers and a Bertrand lens to analyze the pupil plane birefringentmaterials.

SUMMARY

Embodiments of the invention are based, at least in part, on therealization that the various angles of incidence in an interferometer(e.g., having a high NA objective) can be distinguished by thecorresponding spatial frequencies in an interference pattern generatedby scanning the test sample or reference mirror relative to theinterferometer (e.g., towards or away from the objective used to focuslight onto the test sample or reference mirror). Therefore, amathematical spatial frequency decomposition of such an interferencepattern provides access to the relative amplitude and phase of the lightreflected (or scattered) from a sample surface as a function of angle.This knowledge, together with a calibration of the illuminationdistribution in the pupil of the objective and the polarization state ofthe illumination across the pupil plane, provides the multiple-anglereflection (or scattering) amplitude and phase information for everypixel in the field of view, without having to directly image the pupilplane onto a detector array. These multiple-angle data can be used toprovide sample surface characteristics such as thin film thicknessand/or the complex index of refraction on a pixel-by-pixel basis withhigh lateral resolution, simultaneously with surface height profileinformation.

Embodiments of the invention typically include an interferometer, forexample an interference microscope having an interference objective ofthe Mirau, Linnik, Michelson type or the like. The objective illuminatesand collects light from a sample surface over a range of incident anglesφ. For example, φ=0 to 50° for an interference objective having anumerical aperture (NA) of about 0.75. The polarization of theillumination may be radial, linear, circular, field-dependent, oradjustable. Typically, the apparatus further includes a mechanicalscanner for displacing the sample surface along an axis parallel to theoptical axis of the objective (or equivalent motion objective withrespect to the sample) while an electronic camera collects interferenceintensity data for an array of pixels corresponding to field positionson the sample. Alternatively, a reference leg of the interferometer maybe scanned. The result is intensity vs. sample position data for eachpixel for a sequence of objective distances from the sample, stored incomputer memory.

In some embodiments, the computer transforms the interference data foreach pixel into the frequency domain e.g. by Fourier analysis, torecover the magnitude and phase of the constituent spatial frequenciespresent in the interference data. The computer analyzes these data,compares the magnitude and phase to a model representing the surfacestructure of the sample, including incident-angle, polarization and/orwavelength-dependent optical properties of the sample. This analysisdetermines parameters such as surface height and thin film thickness.

Some embodiments select wavelengths or send multiple wavelengths intothe interferometer to perform a detailed analysis of the opticalproperties of materials as function of wavelength, in addition toanalyzing their angle-dependence. Some embodiments analyze the scatteredlight from the sample to determine surface structure information by thediffractive and scattering properties of the surface as a function ofincident angle and wavelength.

Embodiments of the invention include many advantages. For example,embodiments may provide a means for analyzing a surface structure forits optical properties and surface topography simultaneously, e.g., on apixel-by-pixel basis, by frequency-domain decomposition of interferencepatterns generated by vertical scanning of the sample with respect tothe interference objective. Such an approach provides access to theangle-dependent and wavelength-dependent optical properties of thesurface, using both amplitude and phase information from the reflectedlight without the need to directly access the pupil plane of theinstrument.

We now generally summarize different aspects and features of one or moreembodiments of the invention.

In general, in one aspect, the invention features a method including:imaging test light emerging from a test object over a range of angles tointerfere with reference light on a detector, wherein the test andreference light are derived from a common source; for each of theangles, simultaneously varying an optical path length difference fromthe source to the detector between interfering portions of the test andreference light at a rate that depends on the angle at which the testlight emerges from the test object; and determining an angle-dependenceof an optical property of the test object based on the interferencebetween the test and reference light as the optical path lengthdifference is varied for each of the angles.

Embodiments of the method may include any of the following features.

The range of incident angles may correspond to a numerical aperturegreater than 0.7, or more preferably, greater than 0.9.

The detector may be a camera having multiple detector elements and theimaging may include imaging the test light emerging from differentlocations of the test object to corresponding locations on the camera.Furthermore, determining the angle-dependence of the optical propertymay include determining the angle-dependence of the optical property ateach of the different locations of the test object.

The angle-dependence of the optical property may relate to changes inthe optical property as a function of angle of the test light incidenton the test object. The method may further include illuminating multiplelocations of the test object with the test light such that the testlight is incident on each of the multiple locations over the range ofincident angles. In such cases, the illuminating and the imaging mayinvolve a common objective lens. Furthermore, the common source may be aspatially extended source.

In other embodiments, the angle-dependence of the optical propertyrelate to changes in the optical property as a function of angle of thetest light scattered (or diffracted) from the test object. The methodmay further include illuminating multiple locations of the test objectwith the test light having a uniform angle of incidence on the testobject, and wherein the imaging may include imaging test light scatteredover a range of angles from each location of the test object to acorresponding location on the detector. In such cases, the illuminatingand the imaging may involve a common objective lens. Furthermore, thecommon source may be a point source.

The imaging may further include polarizing the test light in a pupilplane of an optical system involved in the imaging.

The method may further include illuminating the test object with thetest light and polarizing the test light in a pupil plane of an opticalsystem used to illuminate the test object.

The common source may be monochromatic. For example, the common sourcemay have a central wavelength and a spectral bandwidth less than 2% ofthe central wavelength.

The simultaneous varying of the optical path length difference for eachof the angles may include moving the test object relative to anobjective used to collect the test light emerging from the test sample.

The simultaneous varying of the optical path length difference for eachof the angles may include moving a reference mirror used to reflect thereference light relative to an objective used to focus the referencelight onto the reference mirror.

The simultaneous varying of the optical path length difference for eachof the angles may include moving a beam splitter positioned within aMirau interference objective.

The simultaneous varying of the optical path length difference for eachof the angles may define a spatial coherence length, and the opticalpath length difference for at least one of the angles may be varied overa range larger than the spatial coherence length.

Determining the angle-dependence of the optical property may include:measuring an interference signal from the detector as the optical pathlength difference is simultaneously varied for each of the angles; andtransforming the interference signal with respect to a coordinatelinearly proportional the optical path length difference for each of theangles to produce a transformed signal that depends on a conjugatevariable to the coordinate. For example, the conjugate variable mayspatial frequency.

The conjugate variable may provide a direct mapping to the angle of testlight incident on, or emerging from, the test object. For example, whenthe conjugate variable is spatial frequency K, the direct mappingbetween the spatial frequency and the angle φ may be given by K(φ)∝cos(φ)/λ, where λ is the wavelength of the test light. For example, whenthe emerging light is reflected from the test sample, the direct mappingbetween the spatial frequency and the angle may be given by K(φ)=4πcos(φ)/λ.

The transformed signal may provide a direct mapping to theangle-dependence of the optical property. For example, thetransformation may correspond to a Fourier transform.

The optical property may be related to the complex reflectivity of thetest object. For example, the optical property may be related to themagnitude of the complex reflectivity of the test object. Also, theoptical property may be related to the phase of the complex reflectivityof the test object.

The angle-dependence of the optical property may be determined based onthe interference between the test and reference light as the opticalpath length difference is varied for each of the angles andprecalibrated angle-dependent characteristics of an optical systeminvolved in the imaging.

The method may further include determining a surface height profile ofthe test object based on the interference between the test and referencelight as the optical path length difference is varied.

The method may further including comparing the angle-dependent changesin the optical property determined from the interference between thetest and reference light to those of a model for the test object. Forexample, the test object may include at least one thin film on asubstrate, and the method may further include determining a thickness ofthe thin film based on the comparison.

In one such embodiment, the optical property includes the magnitude ofthe angle-dependence of the complex reflectivity of the test sample, andthe determination of the thickness of the thin film is based oncomparing the magnitude of the angle-dependence of the complexreflectivity to that of the model. Furthemore, the embodiment mayinclude determining a surface height profile for the test object basedon the comparison. For example, the optical property may further includethe phase of the angle-dependence of the complex reflectivity of thetest sample, and the determination of the surface height profile isbased on the determined thickness of the thin film and comparing thephase of the angle-dependence of the complex reflectivity to that of themodel for the determined thickness.

Finally, the test and reference light may have a first wavelength, andthe method may further include repeating the imaging, varying, anddetermining for test and reference light having a second wavelengthdifferent from the first wavelength.

In general, in another aspect, the invention features a methodincluding: determining an angle-dependence of an optical property of atest object based on scanning interferometry data for the test object.

This method may further include any of the features described above inconnection with the first method.

In general, in yet another aspect, the invention features a methodincluding: imaging test light emerging from a test object over a rangeof angles to interfere with reference light on a detector, wherein thetest and reference light are derived from a monochromatic, common sourceand wherein the test object includes at least one thin film on asubstrate; for each of the angles, simultaneously varying an opticalpath length difference from the source to the detector betweeninterfering portions of the test and reference light at a rate thatdepends on the angle at which the test light emerges from the testobject; and determining a thickness of the thin film based on theinterference between the test and reference light as the optical pathlength difference is varied for each of the angles.

In general, in yet another aspect, the invention features a methodincluding: determining a thickness of a thin film on a test objectincluding the thin film and a substrate supporting the thin film basedon monochromatic scanning interferometry data for the test object.

Embodiments of the third and fourth methods described above may furtherinclude any of the features described above in connection with the firstmethod.

In general, in yet another aspect, the invention features an apparatusincluding: a light source; a detector; a scanning interferometerconfigured to image test light emerging from a test object over a rangeof angles to interfere with reference light on the detector, wherein thetest and reference light are derived from the light source, wherein foreach of the angles, the scanning interferometer is further configured tosimultaneously vary an optical path length difference from the source tothe detector between interfering portions of the test and referencelight at a rate that depends on the angle at which the test lightemerges from the test object; and an electronic processor coupled to thedetector and the scanning interferometer, wherein the electronicprocessor is configured to determine an angle-dependence of an opticalproperty of the test object based on the interference between the testand reference light as the optical path length difference is varied foreach of the angles as measured by the detector.

In general, in yet another aspect, the invention features an apparatusincluding: a monochromatic light source; a detector; a scanninginterferometer configured to image test light emerging from a testobject over a range of angles to interfere with reference light on thedetector, wherein the test and reference light are derived from themonochromatic light source, wherein for each of the angles, the scanninginterferometer is further configured to simultaneously vary an opticalpath length difference from the source to the detector betweeninterfering portions of the test and reference light at a rate thatdepends on the angle at which the test light emerges from the testobject; and an electronic processor coupled to the detector and thescanning interferometer, wherein the electronic processor is configuredto determine a thickness of a thin film on the test object based on theinterference between the test and reference light as the optical pathlength difference is varied for each of the angles.

In general, in yet another aspect, the invention features an apparatusincluding: a scanning interferometry system; and an electronic processorcoupled to the scanning interferometry system, wherein the electronicprocessor is configured to determine an angle-dependence of an opticalproperty of a test object based on scanning interferometry data for thetest object produced by the scanning interferometry system.

In general, in yet another aspect, the invention features an apparatusincluding: a monochromatic scanning interferometry system; and anelectronic processor coupled to the scanning interferometry system,wherein the electronic processor is configured to determine a thicknessof a thin film on the test object based on monochromatic scanninginterferometry data for the test object.

In general, in yet another aspect, the invention features an apparatusincluding: a scanning interferometer configured to image test lightemerging from a test object over a range of angles to interfere withreference light on a detector, wherein the test and reference light arederived from a common source, wherein for each of the angles, thescanning interferometer is further configured to simultaneously vary anoptical path length difference from the source to the detector betweeninterfering portions of the test and reference light at a rate thatdepends on the angle at which the test light emerges from the testobject, wherein the interferometer includes an objective lens positionedto collect the test light emerging from the test object and at least onepolarization optic positioned in a pupil plane of the objective.

For example, the at least one polarization optic may impart apolarization that varies across the pupil plane.

Also, the at least one polarization optic may include a polarizer and atleast one waveplate. For example, the at least one polarization opticmay include two waveplates located a different positions in the pupilplane.

In general, in yet another aspect, the invention features an apparatusincluding: a scanning interferometer configured to image test lightemerging from a test object over a range of angles to interfere withreference light on a detector, wherein the test and reference light arederived from a common source, wherein for each of the angles, thescanning interferometer is further configured to simultaneously vary anoptical path length difference from the source to the detector betweeninterfering portions of the test and reference light at a rate thatdepends on the angle at which the test light emerges from the testobject, wherein the interferometer comprises a source module configuredto illuminate the test object with substantially collimated light. Forexample, the apparatus may further include the common source, and thecommon source may be a monochromatic source.

Furthermore, embodiments of any of the preceding apparatus inventionsmay include any of the corresponding features described above inconnection with the first method. Unless otherwise defined, alltechnical and scientific terms used herein have the same meaning ascommonly understood by one of ordinary skill in the art to which thisinvention belongs. All publications, patent applications, patents, andother references mentioned herein are incorporated by reference in theirentirety. In case of conflict, the present specification, includingdefinitions, will control.

Other features, objects, and advantages of the invention will beapparent from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of a Linnik-type, scanning interferometrysystem.

FIG. 2 is a diagram showing illumination of the test sample through anobjective lens.

FIG. 3 is a diagram of a thin film structure.

FIG. 4 is a simulated interference pattern I(ζ,h) for the structureshown in FIG. 3 built up of 1.8-μm of SiO₂ on Si, using 550-nmmonochromatic light and a 0.9-NA Linnik objective. Note that theinterference signals from both surfaces are mixed together.

FIG. 5 is a simulated interference pattern I(ζ,h) for a simplesingle-surface SiO₂ sample (i.e., no thin films), for comparison withFIG. 4.

FIG. 6 is a graph showing the magnitude Q (φ, h) of the Fouriertransform of the signal in FIG. 4 generated by vertically scanning thethin-film structure of FIG. 3. The spatial frequency relates to incidentangle according to Eq. (4).

FIG. 7 is a graph showing the magnitude Q(φ,h) of the Fourier transformof the signal in FIG. 5 for the single-surface sample. The increasingmagnitude at lower spatial frequencies is the result of increasingreflectivity at shallow angles of incidence.

FIG. 8 is a graph comparing the expected result of P(φ)V₀(φ)√{squareroot over (Z(φ))} for the SiO₂ on Si thin film structure of FIG. 3 forthree film thicknesses in 0.02-μm increments (see Eq. (9)).

FIG. 9 is a graph of the phase α_(Q)(φ,h) as a function of spatialfrequency for the signal in FIG. 4 generated by vertically scanning thethin-film structure of FIG. 3. The spatial frequency relates to incidentangle according to Eq. (4). Note not only the slope of the phase but thedistinctive nonlinearity compared to the simpler single-surfacereflection in FIG. 10.

FIG. 10 is a graph of the phase α_(Q)(φ,h) as a function of spatialfrequency for the signal in FIG. 5 for the single-surface pattern, forcomparison with FIG. 9.

FIG. 11 is a schematic drawing of a Mirau-type, scanning interferometrysystem.

FIG. 12 is a diagram illustrating radial polarization in the pupilplane.

Like reference numerals in different drawings refer to common elements.

DETAILED DESCRIPTION

FIG. 1 shows a scanning interferometer of the Linnik type. Illuminationlight 102 from a source (not shown) is partially transmitted by a beamsplitter 104 to define reference light 106 and partially reflected bybeam splitter 104 to define measurement light 108. The measurement lightis focused by a measurement objective 110 onto a test sample 112 (e.g.,a sample comprising a thin single- or multi-layer film of one or moredissimilar materials). Similarly, the reference light is focused by areference objective 114 onto a reference mirror 116. Preferably, themeasurement and reference objectives have common optical properties(e.g., matched numerical apertures). Measurement light reflected (orscattered or diffracted) from the test sample 112 propagates backthrough measurement objective 110, is transmitted by beam splitter 104,and imaged by imaging lens 118 onto a detector 120. Similarly, referencelight reflected from reference mirror 116 propagates back throughreference objective 114, is reflected by beam splitter 104, and imagedby imaging lens 118 onto a detector 120, where it interferes with themeasurement light.

For simplicity, FIG. 1 shows the measurement and reference lightfocusing onto particular points on the test sample and reference mirror,respectively, and subsequently interfering on a corresponding point onthe detector. Such light corresponds to those portions of theillumination light that propagate perpendicular to the pupil planes forthe measurement and reference legs of the interferometer. Other portionsof the illumination light ultimately illuminate other points on the testsample and reference mirror, which are then imaged onto correspondingpoints on the detector. In FIG. 1, this is illustrated by the dashedlines 122, which correspond to the chief rays emerging from differentpoints on the test sample that are imaged to corresponding points on thedetector. The chief rays intersect in the center of the pupil plane 124of the measurement leg, which is the back focal plane of measurementobjective 110. Light emerging from the test sample at an angle differentfrom that of the chief rays intersect at a different location of pupilplane 124.

In preferred embodiments, detector 120 is a multiple element (i.e.,multi-pixel) camera to independently measure the interference betweenthe measurement and reference light corresponding to different points onthe test sample and reference mirror (i.e., to provide spatialresolution for the interference pattern).

A scanning stage 126 coupled to test sample 112 scans the position ofthe test sample relative to measurement objective 110, as denoted by thescan coordinate ζ in FIG. 1. For example, the scanning stage can bebased on a piezoelectric transducer (PZT). Detector 120 measures theintensity of the optical interference at one or more pixels of thedetector as the relative position of the test sample is being scannedand sends that information to a computer 128 for analysis.

Because the scanning occurs in a region where the measurement light isbeing focused onto the test sample, the scan varies the optical pathlength of the measurement light from the source to the detectordifferently depending on the angle of the measurement light incident on,and emerging from, the test sample. As a result, the optical pathdifference (OPD) from the source to the detector between interferingportions of the measurement and reference light scale differently withthe scan coordinate ζ depending on the angle of the measurement lightincident on, and emerging from, the test sample. In other embodiments ofthe invention, the same result can be achieved by scanning the positionof reference mirror 116 relative to reference objective 114 (instead ofscanning test sample 112 relative to measurement objective 110).

This difference in how OPD varies with the scan coordinate ζ introducesa limited coherence length in the interference signal measured at eachpixel of the detector. For example, the interference signal (as afunction of scan coordinate) is typically modulated by an envelopehaving a spatial coherence length on the order of λ/2(NA)², where λ isthe nominal wavelength of the illumination light and NA is the numericalaperture of the measurement and reference objectives. As describedfurther below, the modulation of the interference signal providesangle-dependent information about the reflectivity of the test sample.To increase the limited spatial coherence, the objectives in thescanning interferometer preferably define a large numerical aperture,e.g., greater than 0.7 (or more preferably, greater than 0.9).

The interference signal can be further modulated by a limited temporalcoherence length associated with the spectral bandwidth of theillumination source. For the present description, however, it is assumedthat the illumination source is nominally monochromatic and anylimitation in temporal coherence is small relative to the limitedspatial coherence. For example, the illumination source may havebandwidth that is less than about 2% of its central wavelength.

Referring again to the Linnik interferometer of FIG. 1, measurementobjective 110 illuminates and views the surface of the test sample overa range of incident angles φ. The interference effect will now becalculated mathematically using a simplified model, assumingmonochromatic illumination. Thereafter, it will be explained how theoptical properties of the sample surface are recovered by mathematicaldecomposition of the interference pattern into its angle-dependentcontributions.

The complex amplitude reflectivity of the surface of the test sample isz(φ) and the corresponding intensity reflectivity Z(φ) isZ(φ)=|z(φ)|².  (1)The phase change on reflection (PCOR) for the sample surface isαz(φ)=arg[z(φ)]  (2),where “arg” in Eq. (2) returns the phase of the complex amplitudereflectivity.

In a simplified scalar (non-polarized) model in which one considers theinterference effects for each angle of incidence separately, theinterference pattern for a single sample point or camera pixel isproportional tog(φ,ζ,h)=R ₀(φ)+Z(φ)+V ₀(φ)√{square root over (Z(φ))}cos[(h−ζ)K(φ)+α₀(φ)+α_(z)(φ)]  (3)where ζ is the scan position (actuated by the PZT) and h is the heightprofile of the sample surface. The parameters R₀(φ), V₀ (φ) and α₀(φ)are DC level, contrast and phase values characteristic of theinterferometer optics, including reference mirror 116, that areindependent of test sample 112. As described further below, acalibration procedure determines these parameters using a known artifactof known optical characteristics. The R₀(φ), V₀(φ) and α₀(φ) parametersmay include a field dependence, as required, to accommodate the opticalproperties of the instrument.

The spatial frequency K(φ) of the interference effect decreases as afunction of angle φ according to

$\begin{matrix}{{{K(\phi)} = {\frac{4\;\pi}{\lambda}{\cos(\phi)}}},} & (4)\end{matrix}$where λ is the illumination wavelength, and it is assumed that themeasurement light is reflected from the test sample (i.e., measurementlight emerges from the test sample at an angle equal to that at which itwas incident on the test sample). Eq. (4) is based on the fact that thescanning is done where the measurement light (or reference light)propagates over a range of angles, and thus the OPD between interferingportions of the measurement and reference light scale differently withthe scan coordinate ζ depending on the angle of the measurement lightincident on the test sample. As a result, Eq. (4) sets forth a uniquerelationship between the spatial frequency in the interference signaland angle of incidence.

Assuming the source light is perfectly incoherent spatially across thepupil and is monochromatic, the net effect of all of the angle-dependentcontributions to the interference phenomenon is given by the incoherentsuperposition integralI(ζ,h)=∫₀ ^(φMAX) g(φ,ζ,h)P(φ)dφ  (5)where φ_(MAX)=arcsin(NA) and the weighting functionP _(φ)=sin(φ)cos(φ)  (6)used in the examples that follow is appropriate for a pupil uniformlyilluminated with light, which is apparent from the consideration of thediagram in FIG. 2 (in which the angle is denoted by ψ rather than φ).

For each pixel, the electronic camera and computer control measure theinterference pattern I(ζ,h) over a range of scan positions ζ. The heighth and the effective reflectivity z(φ) vary across the field and may bedifferent for each pixel.

The unique relationship between spatial frequency and angle of incidenceprovides a means of recovering the individual contributions g(φ,ζ,h) tothe integrated pattern I(ζ,h). The first step is to perform adecomposition of the complete interference pattern, for example, byFourier transformation:q[K(φ),h]=∫ _(−∞) ^(∞) I(ζ,h)exp[iK(φ)ζ]dζ.  (7).The practical requirement of a limited scan truncates the integrationover all ζ in Eq. (7) to a limited range of values that include as muchof the interference signal as required for accurate results. Any othertransform that similarly decomposes the interference pattern may also beused. The transform into a spatial frequency domain is generallyreferred to as frequency domain analysis (FDA).

The decomposition q[K(φ),h] may be interpreted as follows. The zerospatial frequency or DC terms are not separable as a function of angleφ, thereforeq(0,h)=∫₀ ^(φMAX) P(φ)[R ₀(φ)+Z(φ)]dφ.  (8)For all other spatial frequency components having a spatial period muchsmaller than the actual can range in the integration, the magnitude ofq[K(φ),h] isQ(φ,h)=|q[K(φ),h]|=P(φ)V ₀(φ)√{square root over (Z(φ))}  (9)and the complex phase isα_(Q)(φ,h)=arg{q[K(φ),h]}=hK(φ)+α₀(φ)+α_(z)(φ).  (10)

In one embodiment of the invention, the optical system characteristicsα₀(φ), P (φ), V₀(φ) have been determined by prior calibration, e.g., bymeans of a known artifact sample, as was noted in the text accompanyingEq. (3). For example, the measurement can be made with a test samplehaving a known surface height and reflectivity so that the opticalsystem characteristics can be extracted from the Eqs. (9) and (10). Withthe optical system characteristics having been predetermined, Eqs. (9)and (10) provide information on the surface height h and the two opticalproperties Z(φ) and α_(z)(φ) of the surface over the range of incidentangles φ. The optical properties Z(φ) and α_(z)(φ) are themselves oftenlinked by fundamental principles, such as the known optical propertiesof materials and thin films, to specific surface parameters such as filmthickness. Thus these parameters together with the surface height can beadjusted so as to provide the best fit to the measure phase α_(Q)(φ,h)and magnitude Q(φ,h) of q[K(φ),h].

As an example, consider the thin-film structure of FIG. 3. The effectivereflectivity of this structure is given by

$\begin{matrix}{{z(\phi)} = \frac{{r_{1}(\phi)} + {{r_{2}\left( \phi^{\prime} \right)}{\exp\left\lbrack {{\mathbb{i}}\;{K\left( \phi^{\prime} \right)}T} \right\rbrack}}}{1 + {{r_{1}(\phi)}{r_{2}\left( \phi^{\prime} \right)}{\exp\left\lbrack {{\mathbb{i}}\;{K\left( \phi^{\prime} \right)}T} \right\rbrack}}}} & (11)\end{matrix}$where r₁(φ),r₂(φ′) are the reflectivities of the upper and lowersurfaces, respectively, and φ′ is the angle of incidence on the lowersurface calculated from φ and Snell's law. The thin-film Eq. (11)generates distinctive interference effects with a strong dependence onK(φ).

For a quantitative illustration of this example, consider a 1.8-micronfilm of silicon dioxide (SiO₂; index n₁=1.46) on silicon (Si; indexn₂=3.96+0.03i) and an illumination wavelength of 550 nm. The effectivereflectivity z(φ) follows from Eq. (11) and the Fresnel equations forthe reflectivities of the interfaces. A scan of this sample surface withrespect to the interference objective generates an signal such as inFIG. 4. For comparison, FIG. 5 shows a simulated, interference patternI(ζ,h) for a simple single-surface SiO₂ sample (i.e., a thick sample ofSiO₂ with no thin film layer).

After data acquisition, the computer transforms signals similar to thatof FIG. 4 for each image pixel into the frequency domain. The signalsand transforms may differ from pixel to pixel because of fieldvariations in surface topography, optical system parameters, and filmthickness. FIG. 6 shows the magnitude (in this case, the amplitude) ofeach of the constituent spatial frequency contributions to the signal inFIG. 4. This result shows very distinctive features when compared to thefrequency-domain magnitude shown in FIG. 7 generated by a simplesingle-surface structure having the interference signal shown in FIG. 5.

Comparison of FIG. 6 with FIG. 7, for example using FIG. 7 as acalibration, provides an unambiguous determination of the presence of athin film. Further, by comparing FIG. 6 with the theoretical expectationbased on the effective reflectivity of the sample, the computer candetermine, e.g., the thickness of the film assuming the known propertiesof SiO₂ and Si. This is illustrated by FIG. 8, which compares theexpected results of three different films, only one of which (1.80 μm)provides a good match to the Fourier Transformed interference data ofFIG. 6.

A similar analysis is also useful for the phase of the FourierTransform. FIG. 9 and FIG. 10 show the difference between a thin-filmstructure and a simple homogeneous, single-surface sample. Thenonlinearity evident in FIG. 9 is a clear signature of a thin filmeffect. Here again, comparison between measurement and theory providesimportant film thickness information, based on Eq. (10). Furthermore,using the thickness information derived from the amplitude information,one can determine α_(z)(φ) from Eq. (11) and use it in Eq. (10) toextract the surface height variation h among the different pixels.

In other embodiments, an interferometry system different from that inFIG. 1 may be used to provide the scanning interferometry data I(ζ,h) ateach pixel of the camera. For example, the interferometry system may bea Mirau-type interferometer as shown in FIG. 11.

Referring to FIG. 11, a source module 205 provides illumination light206 to a beam splitter 208, which directs it to a Mirau interferometricobjective assembly 210. Assembly 210 includes an objective lens 211, areference flat 212 having a reflective coating on a small centralportion thereof defining a reference mirror 215, and a beam splitter213. During operation, objective lens 211 focuses the illumination lighttowards a test sample 220 through reference flat 212. Beam splitter 213reflects a first portion of the focusing light to reference mirror 215to define reference light 222 and transmits a second portion of thefocusing light to test sample 220 to define measurement light 224. Then,beam splitter 213 recombines the measurement light reflected (orscattered) from test sample 220 with reference light reflected fromreference mirror 215, and objective 211 and imaging lens 230 image thecombined light to interfere on detector (e.g., a multi-pixel camera)240. As in the system of FIG. 1, the measurement signal(s) from thedetector is sent to a computer (not shown).

The scanning in the embodiment of FIG. 11 involves a piezoelectrictransducer (PZT) 260 coupled to Mirau interferometric objective assembly210, which is configured to scan assembly 210 as a whole relative totest sample 220 along the optical axis of objective 211 to provide thescanning interferometry data I(ζ,h) at each pixel of the camera.Alternatively, the PZT may be coupled to the test sample rather thanassembly 210 to provide the relative motion there between, as indicatedby PZT actuator 270. In yet further embodiments, the scanning may beprovided by moving one or both of reference mirror 215 and beam splitter213 relative to objective 211 along the optical axis of objective 211.

Source module 205 includes a spatially extended source 201, a telescopeformed by lenses 202 and 203, and a stop 204 positioned in the frontfocal plane of lens 202 (which coincides with the back focal plane oflens 203). This arrangement images the spatially extended to source ontothe pupil plane 245 of Mirau interferometric objective assembly 210,which is an example of Koehler imaging. The size of stop controls thesize of the illumination field on test sample 220. In other embodiments,the source module may include an arrangement in which a spatiallyextended source is imaged directly onto the test sample, which is knownas critical imaging. Either type of source module may be used with theLinnik-type scanning interferometry system of FIG. 1.

In further embodiments, the scanning interferometer can be of theMichelson-type.

In further embodiments of the invention, the scanning interferometrysystem may used to determine angle-dependent scattering or diffractioninformation about a test sample, i.e., for scatterometry. For example,the scanning interferometry system may be used to illuminate a testsample with test light incident over only a very narrow range ofincident angles (e.g., substantially normal incidence or otherwisecollimated), which may then be scattered or diffracted by the testsample. The light emerging from the sample is imaged to a camera tointerfere with reference light as described above. As with the reflectedlight in the embodiments described above, the spatial frequency of eachcomponent in the scanning interferometry signal will depend vary withangle of the test light emerging from the test sample. For substantiallynormal incidence, the spatial frequency varies according to:

$\begin{matrix}{{{K(\phi)} = {\frac{2\;\pi}{\lambda}{\cos(\phi)}}},} & (12)\end{matrix}$

which differs from Eq. (4) be a factor of 2 because of the normalincidence. The other parts of the mathematical analysis remainunchanged, however, and the scanning interferometry data I(ζ,h) from ascattering or diffractive test sample can be analyzed according to Eqs.(7)-(10) to provide the angle-dependent, phase and amplitudescattering/diffraction coefficients for the test sample. Thus, avertical scan (i.e., a scan along the optical axis of an objective)followed by Fourier analysis allows for a measurement of diffractedand/or scattered light as a function of emerging angle, without directlyaccessing or imaging the back focal plane of the objective. Moreover, asabove, the angle-dependence of such optical properties can be determinedlocally over an area of the test sample based on the resolution of theimaging system and the camera pixel size. To provide the substantiallynormal incidence illumination, for example, the source module can beconfigured to image a point source onto the pupil plane or to otherwisedecrease the degree to which the illumination light fills the numericalaperature of the measurement objective. The scatterometry technique maybe useful for resolving discrete structures in the sample surface, suchas grating lines, edges, or general surface roughness, which maydiffract and/or scatter light to higher angles.

In the above embodiments, it has been assumed that the polarizationstate of the light in the pupil plane is random, i.e., comprised ofapproximately equal amounts of both s polarizations (orthogonal to theplane of incidence) and p (orthogonal to the plane of incidence)polarizations. Alternative polarizations are possible, including pure spolarization, such as may be realized by means of a radial polarizerplaced in the pupil plane (e.g., in the back-focal plane of themeasurement object in the case of a Linnik interferometer and in theback focal plane of the common objective in the Mirau interferometer).Such radial polarization is illustrated in FIG. 12. Other possiblepolarizations include radial p polarization, circular polarization, andmodulated (e.g. two states, one following the other) polarization forellipsometric measurements. In other words, optical properties of thetest sample can be resolved not only with respect to their angledependence, but also with respect to their polarization dependence orwith respect to a selected polarization. Such information may also beused to improve the accuracy of thin film structure characterization.

To provide such ellipsometry measurements, the scanning interferometrysystem may include a fixed or variable polarizer in the pupil plane.Referring again to FIG. 11, the Mirau-type interferometry system, forexample, includes polarization optics 280 in the pupil plane to select adesired polarization for the ligh incident on, and emerging from thetest sample. Furthermore, the polarization optics may be reconfigurableto vary the selected polarization. The polarization optics may includeone or more elements including polarizers, waveplates, apodizationapertures, and/or modulation elements for selecting a givenpolarization. Furthermore, the polarization optics may be fixed,structured or reconfigurable, for the purpose of generating data similarto that of an ellipsometer. For example, a first measurement with aradially-polarized pupil for s polarization, followed by aradially-polarized pupil for p polarization. In another example, one mayuse an apodized pupil plane with linearly polarized light, e.g., a slitor wedge, which can be rotated in the pupil plane so as to direct anydesired linear polarization state to the object, or a reconfigurablescreen such as a liquid crystal display.

Moreover, the polarization optics may provide a variable polarizationacross the pupil plane (e.g., by including multiple polarizers or aspatial modulator). Thus, one can “tag” the polarization state accordingto spatial frequency, for example, by providing a different polarizationfor high angles of incidence than shallow angles.

In yet further embodiments, the selectable polarization may be combinedwith a phase shift as a function of polarization. For example, thepolarization optics may include a linear polarizer is positioned in thepupil plane and followed by two waveplates (e.g., eighth-wave plates) inopposing quadrants of the pupil plane. The linear polarization resultsin a full range of polarization angles with respect to the incidentplanes of the objective. If the waveplates are aligned so that, forexample, the predominately s-polarized light has a fixed phase shift,then both radial s polarized and p polarized light are presentsimultaneously, but shifted in phase with respect to each other, e.g.,by pi, so that the interferometer is effectively detecting thedifference between these two polarization states as the fundamentalsignal.

As described above, placing the polarization optics in the pupil planeallows for various angle-resolved type polarization measurements. Infurther embodiments, however, polarization optics may be positionedelsewhere in the apparatus. For example, linear polarization can beachieved anywhere in the system.

In further embodiments, any of the reflectometry, scatterometry, andellipsometry techniques described above may be repeated sequentially fordifferent wavelengths to provide the wavelength dependence of the sampleoptical properties of interest. Such information may be used for fittingmore complex surface models.

Other embodiments of the invention may include broadband illumination.For example, the illumination may be broadband, as is common in, e.g.,white light interference microscopes. This increases the amount ofinformation to which the computer may find the best fit for a complexsurface model.

The light source for the scanning interferometry systems may be any of,for example, a laser, a laser diode, a light-emitting diode, a filteredincandescent source, and an arc lamp.

The methods and systems described above can be particularly useful insemiconductor applications. Additional embodiments of the inventioninclude applying any of the measurement techniques described above toaddress any of the semiconductor applications described below.

It is presently of considerable interest in the semiconductor industryto make quantitative measurements of surface topography. Due to thesmall size of typical chip features, the instruments used to make thesemeasurements typically must have high spatial resolution both paralleland perpendicular to the chip surface. Engineers and scientists usesurface topography measuring systems for process control and to detectdefects that occur in the course of manufacturing, especially as aresult of processes such as etching, polishing, cleaning and patterning.

For process control and defect detection to be particularly useful, asurface topography measuring system should have lateral resolutioncomparable to the lateral size of typical surface features, and verticalresolution comparable to the minimum allowed surface step height.Typically, this requires a lateral resolution of less than a micron, anda vertical resolution of less than 1 nanometer. It is also preferablefor such a system to make its measurements without contacting thesurface of the chip, or otherwise exerting a potentially damaging forceupon it, so as to avoid modifying the surface or introducing defects.Further, as it is well-known that the effects of many processes used inchip making depend strongly on local factors such as pattern density andedge proximity, it is also important for a surface topography measuringsystem to have high measuring throughput, and the ability to sampledensely over large areas in regions which may contain one or manysurface features of interest.

It is becoming common among chip makers to use the so-called ‘dualdamascene copper’ process to fabricate electrical interconnects betweendifferent parts of a chip. This is an example of a process which may beeffectively characterized using a suitable surface topography system.The dual damascene process may be considered to have five parts: (1) aninterlayer dielectric (ILD) deposition, in which a layer of dielectricmaterial (such as a polymer, or glass) is deposited onto the surface ofa wafer (containing a plurality of individual chips); (2) chemicalmechanical polishing (CMP), in which the dielectric layer is polished soas to create a smooth surface, suitable for precision opticallithography, (3) a combination of lithographic patterning and reactiveion etching steps, in which a complex network is created comprisingnarrow trenches running parallel to the wafer surface and small viasrunning from the bottom of the trenches to a lower (previously defined)electrically conducting layer, (4) a combination of metal depositionsteps which result in the trenches and vias being over-filled withcopper, and (5) a final chemical mechanical polishing (CMP) step inwhich the excess copper is removed, leaving a network of copper filledtrenches (and possibly vias) surrounded by dielectric material.

Typically the thickness of the copper in the trench areas (i.e., thetrench depth), and the thickness of the surrounding dielectric lie in arange of 0.2 to 0.5 microns. The width of the resulting trenches may bein a range of from 100 to 100,000 nanometers, and the copper regionswithin each chip may in some regions form regular patterns such asarrays of parallel lines, and in others they may have no apparentpattern. Likewise, within some regions the surface may be denselycovered with copper regions, and in other regions, the copper regionsmay be sparse. It is important to appreciate that the polishing rate,and therefore the remaining copper (and dielectric) thickness afterpolishing, depends strongly and in a complex manner on the polishingconditions (such as the pad pressure and polishing slurry composition),as well as on the local detailed arrangement (i.e., orientation,proximity and shape) of copper and surrounding dielectric regions.

This ‘position dependent polishing rate’ is known to give rise tovariable surface topography on many lateral length scales. For example,it may mean that chips located closer to the edge of a wafer onaggregate are polished more rapidly than those located close to thecenter, creating copper regions which are thinner than desired near theedges, and thicker than desired at the center. This is an example of a‘wafer scale’ process nonuniformity—i.e., one occurring on length scalecomparable to the wafer diameter. It is also known that regions whichhave a high density of copper trenches polish at a higher rate thannearby regions with low copper line densities. This leads to aphenomenon known as ‘CMP induced erosion’ in the high copper densityregions. This is an example of a ‘chip scale’ processnonuniformity—i.e., one occurring on a length scale comparable to (andsometimes much less than) the linear dimensions of a single chip.Another type of chip scale nonuniformity, known as ‘dishing’, occurswithin single copper filled trench regions (which tend to polish at ahigher rate than the surrounding dielectric material). For trenchesgreater than a few microns in width dishing may become severe with theresult that affected lines later exhibit excessive electricalresistance, leading to a chip failure.

CMP induced wafer and chip scale process nonuniformities are inherentlydifficult to predict, and they are subject to change over time asconditions within the CMP processing system evolve. To effectivelymonitor, and suitably adjust the process conditions for the purpose ofensuring that any nonuniformities remain within acceptable limits, it isimportant for process engineers to make frequent non-contact surfacetopography measurements on chips at a large number and wide variety oflocations. This is possible using embodiments of the interferometrytechniques described above.

More generally, the interferometry techniques described above may usedfor any of the following surface analysis problems: simple thin films;multilayer thin films; sharp edges and surface features that diffract orotherwise generate complex interference effects; unresolved surfaceroughness; unresolved surface features, for example, a sub-wavelengthwidth groove on an otherwise smooth surface; dissimilar materials;polarization-dependent properties of the surface; and deflections,vibrations or motions of the surface or deformable surface features thatresult in incident-angle dependent perturbations of the interferencephenomenon. For the case of simple thin films, the variable parameter ofinterest may be the film thickness, the refractive index of the film,the refractive index of the substrate, or some combination thereof. Forthe case of dissimilar materials, for example, the surface may comprisea combination of thin film and a solid metal, and a fit of theangle-dependent surface properties would be made to a library oftheoretical predictions which would include both surface structure typesto automatically identify the film or the solid metal by a match to thecorresponding interference intensity signal

Any of the computer analysis methods described above can be implementedin hardware or software, or a combination of both. The methods can beimplemented in computer programs using standard programming techniquesfollowing the method and figures described herein. Program code isapplied to input data to perform the functions described herein andgenerate output information. The output information is applied to one ormore output devices such as a display monitor. Each program may beimplemented in a high level procedural or object oriented programminglanguage to communicate with a computer system. However, the programscan be implemented in assembly or machine language, if desired. In anycase, the language can be a compiled or interpreted language. Moreover,the program can run on dedicated integrated circuits preprogrammed forthat purpose.

Each such computer program is preferably stored on a storage medium ordevice (e.g., ROM or magnetic diskette) readable by a general or specialpurpose programmable computer, for configuring and operating thecomputer when the storage media or device is read by the computer toperform the procedures described herein. The computer program can alsoreside in cache or main memory during program execution. The analysismethod can also be implemented as a computer-readable storage medium,configured with a computer program, where the storage medium soconfigured causes a computer to operate in a specific and predefinedmanner to perform the functions described herein.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.

1. A method comprising: collecting test light emerging from a testobject to interfere with reference light on a detector, wherein the testand reference light are derived from a common source; varying an opticalpath length difference from the source to the detector betweeninterfering portions of the test and reference light; for each ofmultiple surface locations of the test object, calculating multiplefunctions based on an interference signal corresponding to theinterference between the test light emerging from the correspondingsurface location of the test object and the reference light as theoptical path length difference is varied, wherein each function has amulti-valued output; and comparing the multi-valued output of each ofthe multiple functions for each of the surface locations to multi-valuedinformation derived from a model to collectively determine a best-fitvalue for each of one or more model parameters and thereby yieldinformation about the test object.
 2. The method of claim 1, wherein themultiple functions comprise an amplitude function and a phase function.3. The method of claim 2, where the amplitude and phase functionscorrespond to amplitude and phase components of a transform of theinterference signal.
 4. The method of claim 3, wherein the transform isa Fourier transform.
 5. The method of claim 1, wherein the multiplefunctions are each functions of at least one coordinate comprising aspatial frequency coordinate.
 6. The method of claim 5, wherein thespatial frequency coordinate has a one-to-one mapping to an anglecoordinate for the test light emerging from the test object.
 7. Themethod of claim 1, wherein the multiple functions are each functions ofat least one coordinate comprising a wavelength coordinate.
 8. Themethod of claim 1, wherein the test object comprises a layer and the oneor more model parameters comprise a thickness for the layer.
 9. Themethod of claim 1, wherein the test object comprises a grating and theone or more model parameter comprise a property of the grating.
 10. Themethod of claim 1, further comprising outputting information about thebest-fit values for the model parameters and wherein the test object isa component in a semiconductor manufacturing process and the methodfurther comprises characterizing the semiconductor manufacturing processbased on the information about the best fit values for the modelparameters.
 11. The method of claim 1, wherein the detector is a camerahaving multiple detector elements and the collecting comprises imagingthe test light emerging from different locations of the test object tocorresponding locations on the camera.
 12. The method of claim 1,wherein the test light is collected over a range of angles thatcorrespond to a numerical aperture of 0.75 or greater.
 13. The method ofclaim 1, wherein the test light is collected over a range of angles thatcorrespond to a numerical aperture greater than 0.7.
 14. The method ofclaim 1, wherein the test light is collected over a range of angles thatcorrespond to a numerical aperture greater than 0.9.
 15. The method ofclaim 1, wherein the collecting further comprises polarizing the testlight before interfering it with the reference light on the detector.16. The method of claim 1, wherein the optical path length difference isvaried over a range larger than a coherence length for an interferometrysystem used to measure the interference signal.
 17. The method of claim16, wherein the coherence length is based on a range of wavelengths anda range of collection angles for the test light.
 18. The method of claim1, wherein the calculating and comparing is repeated for each ofmultiple interference signals corresponding to different transverselocations of the test object.
 19. An apparatus comprising: a lightsource; a detector; a scanning interferometer configured to collect testlight emerging from a test object to interfere with reference light onthe detector, wherein the test and reference light are derived from thelight source; and an electronic processor coupled to the detector andthe scanning interferometer, wherein the electronic processor isconfigured to: i) cause the scanning interferometer to vary an opticalpath length difference from the source to the detector betweeninterfering portions of the test and reference light; ii) for each ofmultiple surface locations of the test object, cause the detector tomeasure at least one interference signal corresponding to theinterference between the test light emerging from the correspondingsurface location of the test object and the reference light as theoptical path length difference is varied; iii) for each of the multiplesurface locations of the test object, calculate multiple functions basedon the corresponding interference signal, wherein each function has amulti-valued output; and iv) compare the multi-valued output of each ofthe multiple functions for each of the surface locations to multi-valuedinformation derived from a model to collectively determine a best-fitvalue for each of one or more model parameters for the test object andthereby yield information about the test object.
 20. The apparatus ofclaim 19, wherein the multiple functions comprise an amplitude functionand a phase function.
 21. The apparatus of claim 19, where the amplitudeand phase functions correspond to amplitude and phase components of atransform of the interference signal.
 22. The apparatus of claim 21,wherein the transform is a Fourier transform.
 23. The apparatus of claim19, wherein the multiple functions are each functions of at least onecoordinate comprising a spatial frequency coordinate.
 24. The apparatusof claim 23, wherein the spatial frequency coordinate has a one-to-onemapping to an angle coordinate for the test light emerging from the testobject.
 25. The apparatus of claim 19, wherein the multiple functionsare each functions of at least one coordinate comprising a wavelengthcoordinate.
 26. The apparatus of claim 19, wherein the processor isfurther configured to output information.
 27. The apparatus of claim 19,wherein the detector is a camera having multiple detector elements andthe interferometer is configured to image the test light emerging fromdifferent locations of the test object to corresponding locations on thecamera.
 28. The apparatus of claim 19, wherein the interferometer isconfigured to collect the test light over a range of angles thatcorrespond to a numerical aperture of 0.75 or greater.
 29. The apparatusof claim 19, wherein the interferometer is configured to collect thetest light over a range of angles that correspond to a numericalaperture greater than 0.7.
 30. The apparatus of claim 19, wherein theinterferometer is configured to collect the test light over a range ofangles that correspond to a numerical aperture greater than 0.9.
 31. Theapparatus of claim 19, wherein the interferometer is further configuredto polarize the test light before interfering it with the referencelight on the detector.
 32. The apparatus of claim 19, wherein theinterferometer is configured to vary the optical path length differenceover a range larger than a coherence length for the interferometer. 33.The apparatus of claim 19, wherein the coherence length is based on arange of wavelengths and a range of collection angles for the testlight.
 34. The method of claim 19, wherein the electronic processor isfurther configured to perform the calculating and comparing for each ofmultiple interference signals corresponding to different transverselocations of the test object.